The present invention relates to an intermediate anchor for centrifugal concrete molds composed of tubular semi-shells with end clamping heads, for producing conically extending tubular pre-stressed concrete bodies of significant length, such as for example, pre-stressed concrete masts for high voltage power lines, light poles for deep bowl reflectors and the like.
During producing of conically extending tubular pre-stressed concrete masts it is known for example from U.S. Pat. No. 3,738,786, to extend only a part of the pretensioning wires or pretensioning strands through the whole length of the mast, while the remaining pretensioning wires or pretensioning masts extend from the foot of the mast only over a portion of the mast length. These features are required in the pretensioned concrete mass of a great length of for example 30 meters or more so that the pressure which is applied from the pretensioning wires on the upwardly conically reducing pretensioned concrete body with reducing concrete crosssection, does not exceed a permissible maximum value per cross-sectional surface. Moreover, it is necessary to take care that the pretensioning wires or pretensioning strands in the conically reducing upper mast portion are not located too close to one another therefore the concrete between the wires or strands can be uniformly distributed. Due to the shortened pretensioning wires or pretensioning strands also a significant material saving in the upper mast region is achieved.
For producing such prestressed concrete masts, the used centrifugal concrete molds conventionally composed of two conically extending semi-shells must be provided with several intermediate anchors at corresponding distances from the foot-side end for the mold rotatable about a horizontal axis. Thereby the shortened pretensioning wires or pretensioning strands and the throughgoing pretensioning wires or pretensioning strands can be held under the required pretensioning.
Since such shortened prestressing wires or prestressing strands are needed as a rule only for the conically extending pretensioned concrete mass of a great length, the pretensioning wires or pretensioning strands conventionally have a diameter of approximately 8 mm or more. Pretensioning wires or pretensioning strands with such diameters cannot be bent manually, and moreover special bending tools are needed for this purpose which has been shown to be disadvantageous for the treatment of pretensioning wires or pretensioning strands.
An intermediate anchor in accordance with an especially simple embodiment is known, which is formed as a substantially circular segment-shaped rounded anchoring plate. The anchoring plate extends through a longitudinal slot in the wall of both semi-shells of the centrifugal concrete mold provided with a longitudinal groove or depression on their circular edge for inserting bent ends of a shortened pretensioning wire. The outwardly guided end of the pretensioning wire is provided with an anchoring element supported outside on the anchoring plate. The anchoring plate is held on the wall of the centrifugal concrete mold by flange parts extending at both sides of the longitudinal slot and secured by two mounting pins extending through the anchoring plate and both parallel flange parts.
Such an intermediate anchor which on first flange is simple, possesses however, some disadvantages since the shortened pretensioning wires or pretensioning strands must be bent for mounting on the respective anchoring plates. Only for the anchoring on the lower semi-cell of the centrifugal concrete mold these tensioning wires can be preliminarily cut to such a length as required for the anchoring of the intermediate anchor up to the anchoring of the foot-side clamping head of the centrifugal concrete mold but not in the upper part of the centrifugal concrete mold. There the shortened pretensioning wires must have a significant length surplus so that the foreclosing of the upper mold they can be guided from the inner side to the intermediate anchor locations outwardly by hand. Since during the fast binding of the concrete mixture for the closing of the centrifugal concrete mold and the subsequent centrifugal process only a relatively short time is available, the preparation and mounting works must take a lot of effort resulting in high manufacturing costs.
A further disadvantage is that the longitudinal slots provided for the mounting of the anchoring plates on the intermediate locations between the outwardly directed parallel flange parts can be sealed only in imperfect manner. These imperfect sealings lead during the centrifugal process to discharge of cement mass through the slots provided near the anchor plates and leading the pretensioning wire ends outwardly under the action of the centrifugal forces. The discharge substantially contaminates the whole centrifugal concrete installation and due to fast binding cannot be practically removed without residues. Such untighteness of the passages for the shortened pretensioning wires or pretensioning strands can also lead to an imperfect binding of the accelerated concrete in these intermediate anchor regions.
Similar differences occur in another intermediate anchor, in which the pretensioning wire ends in the region of the intermediate anchor are not guided through the wall of the centrifugal concrete mold outwardly but instead anchored in the interior of the centrifugal concrete mold on the anchoring plates. The anchoring plates similarly to the above described intermediate anchoring, are also mounted in the longitudinal slots on the centrifugal concrete mold between the outwardly extending parallel flange parts.
In the intermediate anchor in accordance with a further embodiment shown in FIGS. 8, 9 of U.S. Pat. No. 3,738,786, the anchoring plate extending perpendicularly into the hollow chamber of the centrifugal concrete mold must serve as an abutment for a semi-circular holding plate with a peripheral groove for inserting the pretensioning wire or the pretensioning strand with an end-side loop. The formation of the pretensioning wire or the pretensioning strand with such a loop requires for the pretensioning wires or pretensioning strands with diameters of at least 1-2 inches or more than 12 mm, a significant bending expense for preparation of such pretensioning wires or pretensioning strands. These problems are so great that they are not used in practice for economical reasons.
Finally, a further embodiment of the intermediate anchor shown in FIGS. 10 and 11 of U.S. Pat. No. 3,738,786 deals with the anchoring plate which extends inwardly through a longitudinal slot of the centrifugal concrete mold as in the previous embodiments and secured by two pins on the flanges which extend parallel to it. In this embodiment the anchoring plate engages the pretensioning wire or the pretensioning strand in a fork-shaped manner and has a perpendicular abutment surface for a special clamping element. The clamping element is mounted on the end of the pretensioning wire or the pretensioning strand inside the centrifugal concrete mold and secured the pretensioning wire or the pretensioning strand on the anchoring plate as an abutment body.
In this embodiment an intermediate anchoring possesses the same above mentioned sealing difficulties. Moreover, the special anchoring required for the shortened pretensioning wires or pretensioning strands and abutting against the anchoring plate of the intermediate anchor is lost after the removal of the pretensioned concrete body, since the anchoring plate can be removed from the recess in the wall of the pretensioned concrete body, while, however the clamping anchoring which in addition to the recess is fixedly anchored in the wall of the pretensioned concrete body, cannot be removed.